The goal of this application is to use the zebrafish to identify novel pathways that regulate stem cell self- renewal in both normal muscle and embryonal rhabdomyosarcoma (ERMS), a pediatric malignancy of muscle. The zebrafish, with its close synteny to the human genome and its conserved molecular pathways regulating the development of tissues and organs, offers a powerful tool with which to conduct such research. My hypothesis is that evolutionary conserved pathways regulate stem cell function in both normal satellite cells and cancer stem cells found in ERMS. In normal muscle, satellite cells divide asymmetrically producing a differentiated daughter cell and another satellite cell, ultimately allowing for the regeneration muscle fibers following injury. By contrast, activation of self-renewal pathways in cancer is often deleterious. For example, a subset of tumor cells are capable of producing differentiated cell types and yet retain the capacity for self-renewal. It is these cancer stem cells that must be targeted for destruction if patients are to remain tumor free following conventional chemotherapeutic and radiation treatments. In Aim 1, normal and cancer stem cell populations will be identified within zebrafish muscle. Transgenic approaches that label normal muscle cell populations based on differentiation status have previously been established as has a robust zebrafish transgenic model of RAS-induced rhabdomyosarcoma that is molecularly similar to human disease. The stem cell populations in normal muscle and ERMS will be identified using these reagents and expression analysis of these cell populations will be performed to assess whether conserved genetic programs are associated with differentiation status and self-renewal in both cell types. In Aim 2, the function of these evolutionary conserved gene factors will be interrogated, establishing a role for these genes in self- renewal of muscle stem cells. Specifically, a targeted genetic modifier screen will be performed to uncover regulators of self-renewal and stem cell number in ERMS. Further experiments will assess if these factors also regulate satellite cell number and function. The long-term goal of this application is to discover new genetic pathways that both increase normal satellite cells in the case of muscle degenerative disease or decrease self-renewal potential in cancer. Ultimately, a subset of these genetic modifiers of self-renewal may prove to be useful targets for small molecule intervention in the treatment of human disease.